CSCS Supercomputers help searching for topological defects in space

Topological defects in space may have developed fractions of a second after the Big Bang. Simulations of these wormlike entities and a comparison of the simulations with cosmic background radiation measurements by the Planck satellite should confirm their existence.

by Simone Ulmer, CSCS

When Martin Kunz, a professor in the Cosmology Group » at the University of Geneva, and his team at the CSCS commandeer “Monte Rosa”, they mean business. Hardly any other computer job gets a look-in on the CSCS supercomputer as it works to ninety per cent of its capacity. Hardly surprising when you consider that the scientists are looking to simulate a particular moment in the origin of the universe when complex mechanisms and all manner of scales played a key role: the point when topological defects, so-called “cosmic strings” to be precise, might have formed.

Smaller than a trillionth the size of a hydrogen atom

Topological defects are a common phenomenon in solid-state physics: if an area of a crystal does not undergo the phase transition, in other words reorganisation in the crystal lattice, during crystallisation, a topological defect develops in the lattice. To put it simply, in cosmology topological defects are supposed to form where a phase transition occurs in the universe. This was supposedly the case a few fractions of a second after the Big Bang. The phase transition caused different basic states to develop in different areas of the cosmos. Wherever the different field conditions ultimately met, topological defects such as cosmic strings were able to form. They are supposed to be less than a trillionth the size of a hydrogen atom in diameter and quasi one-dimensional – “quasi” because, despite their defined one-dimensionality, they supposedly extend through space almost infinitely and as extremely thin “worms”. In theory, these worms weigh about twenty per cent of the Earth’s mass per kilometre in length.

Cosmic strings: A view through the simulated universe, gained at the CSCS supercomputer “Monte Rosa”. The image shows cosmic strings (in grey) and the field density (in colour on a plane through the simulation box). Most of the strings form a tangled network, but they can also build string loops that were chopped off the long strings through intersections. (Image: David Daverio, Université de Genève, using simulation data obtained at the CSCS.

Resurgence of cosmic strings

Kunz already examined topological defects in his dissertation under Geneva cosmology professor Ruth Durrer, where he endeavoured to use them to explain where the fluctuations one sees in the cosmic background radiation and out of which structures such as galaxies or galaxy clusters develop come from. However, his and other studies revealed that this does not work. “After that, topological defects were dead and buried in cosmology for a while,” says the Geneva professor. It was not until cosmic inflation models became ingrained in the theories of particle physics that cosmologists began to focus on cosmic strings again. “People realised that cosmic strings are mostly produced when inflation is integrated in standard particle physics,” says Kunz. “The existence of the defects can thus be motivated far more effectively with inflation.”

During the inflation phase, the young universe is supposed to have expanded extremely quickly at an exponentially increasing speed. In the models used, the phase of rapid expansion mostly ends with a phase transition that also leads to so-called symmetry breaking. Here, the basic forces of the strong and weak interaction and the electromagnetic force are supposed to have split into forces acting separately. According to the so-called Grand Unified Theory (GUT), these three basic forces had previously manifested themselves in a single force.

In their models, Martin Kunz and his team simulate cosmic strings during this phase transition. After all, it provides the ideal conditions for producing detectable cosmic strings as, at the time of the GUT symmetry breaking in the universe, an incredibly high energy level of 1016 gigaelectron volts prevailed – one billion times more energy than the large hadron collider at Cern can generate. According to the researchers, at such energy levels the defects are so strong that they should be detectable.

To detect the cosmic strings, the team thus simulates their location and distribution in different field conditions and measures their energy density. The results of the simulations are channelled into a programme that calculates the corresponding fluctuations in the cosmic background radiation using linearised equations. By comparing the results with the cosmic background radiation data, which the Planck satellite supplies during its mission, the researchers are hoping to be able to confirm these unusual defects. The cosmic background radiation may well only have originated 380,000 years after the Big Bang; however, it exhibits minimal fluctuations that, from what we know today, stem from the early universe. “If topological defects are present in the universe, they produce additional fluctuations that effectively contain a finger print of the defects and thus allow conclusions to be drawn as to their nature and origin,” says Kunz.

Moment of truth draws closer

Martin Kunz’s doctoral student David Daverio helped to improve the complex simulations significantly. In his Master’s thesis, he completely rewrote the parallelisation of the codes used, which now means that the simulations can be carried out considerably more efficiently and a resolution that is sixty-four times higher achieved. This is an advantage for the researchers as time is of the essence. By 2014 at the latest, the results of the simulations need to be on hand so that they can be compared with the new Planck data. If the theories on the links between inflation, the formation of cosmic strings and the existence of a GUT are correct, this should be proved from the Planck data by then at the latest. Otherwise, once again it will be a case of excluding a model and trying to detect the supposed cosmic strings in another way – or bury them once and for all.